Strange balanced modulator design question

Strange balanced modulator design question

Post by David Feldm » Mon, 16 Sep 1996 04:00:00

I am looking at schematic of a Japanese SSB transceiver. It has a type of
balanced modulator I've never seen before. The audio from the mic is
amplified once by a conventional class A transistor amp, then fed into
an IC. The IC has a 5 MHz crystal. Based on the block diagram, the
purpose of the IC is to take 300-2700 Hz audio and shift it to 4400-2000
Hz, before it is fed into a balanced modulator using four diodes.

Does anyone have an idea why they would shift and invert the audio
passband before sending it to the balanced modulator? There are easier
ways of getting the opposite sideband (move the BFO crystal relative to
the filter) and I can't imagine why they'd want to shift the audio up,
except maybe to improve opposite sideband supression (it's a very simple
4 pole crystal ladder filter). Could the sideband inversion just be an
artifact of the intended frequency shift? One clue is they move the BFO
(by changing load on the bfo crystal) about 3800 Hz when going between TX
and RX modes. The audio processor chip I'm referring to is only in the TX
audio path - the RX audio path is completely conventional (it's a transceiver).

The chip doing the magic is a M64021, and I have no other specs on it.

Any comments?


 
 
 

Strange balanced modulator design question

Post by Richard Karlqui » Mon, 16 Sep 1996 04:00:00



>I am looking at schematic of a Japanese SSB transceiver. It has a type of
>balanced modulator I've never seen before. The audio from the mic is
>amplified once by a conventional class A transistor amp, then fed into
>an IC. The IC has a 5 MHz crystal. Based on the block diagram, the
>purpose of the IC is to take 300-2700 Hz audio and shift it to 4400-2000
>Hz, before it is fed into a balanced modulator using four diodes.

>Does anyone have an idea why they would shift and invert the audio
>passband before sending it to the balanced modulator? There are easier

This is just a guess, but they may be using an asymmetrical SSB
filter, in which case you can't simply shift the BFO, you have to
flip the sidebands elsewhere.  

Rick Karlquist N6RK

 
 
 

Strange balanced modulator design question

Post by Rocc » Mon, 16 Sep 1996 04:00:00

the original audio passband has been shifted up 4700 Hz

I should have typed "shifted up 1700 Hz".

Joe
WA3CMQ

 
 
 

Strange balanced modulator design question

Post by Rocc » Mon, 16 Sep 1996 04:00:00


> I am looking at schematic of a Japanese SSB transceiver. It has a type of
> balanced modulator I've never seen before. The audio from the mic is
> amplified once by a conventional class A transistor amp, then fed into
> an IC. The IC has a 5 MHz crystal. Based on the block diagram, the
> purpose of the IC is to take 300-2700 Hz audio and shift it to 4400-2000
> Hz, before it is fed into a balanced modulator using four diodes.

If this is what it seems, it's pretty clever. Seems that the 300-2700 hz
audio is mixed with a 4700HZ signal to get a difference product of
4400-2000 HZ...the original audio passband has been shifted up 4700 Hz
and frequency inverted. The crystal filter used for sideband suppression
now can be very simple, since its job is much easier. The upper and
lower sidebands are now 4000 Hz apart at their closest points, instead
of 600 Hz. A simple and relatively sloppy filter can easily filter out
the unwanted sideband. The audio inversion is easily fixed by shifting
the operating frequency to the opposite side of carrier relative to
where it originally was...hence the 3800 Hz carrier shift on transmit.

Joe
WA3CMQ

 
 
 

Strange balanced modulator design question

Post by Jake Brodsk » Tue, 17 Sep 1996 04:00:00

I'm just speculating as to what that modulator circuit might be like...

If complexity and expense were no object, why not run the audio through
an audio band pass filter and then through an A/D sampler (aka mixer)
to put the signal at 4000 to 7100 Hz, or something like that.  Then,
using more filtering (switched capacitor?) and the phasing method of
SSB, one could make a nice, low distortion signal.  Could this be what
those clever Japanese were doing?

73,



Amateur Radio Station AB3A
"Beware of the massive impossible!"

 
 
 

Strange balanced modulator design question

Post by David Feldm » Tue, 17 Sep 1996 04:00:00


>I'm just speculating as to what that modulator circuit might be like...

>If complexity and expense were no object, why not run the audio through
>an audio band pass filter and then through an A/D sampler (aka mixer)
>to put the signal at 4000 to 7100 Hz, or something like that.  Then,
>using more filtering (switched capacitor?) and the phasing method of
>SSB, one could make a nice, low distortion signal.  Could this be what
>those clever Japanese were doing?

I suspect it's some of this... The radio is a 144 MHz SSB HT, and based
on my friend's evaluation in Japan, he reported that the transmitted audio
quality was considered unusually good (describing it as a "human voice",
as opposed to a "robot voice" which afflicts most SSB transmissions). I
suspect the primary goal was to improve opposite sideband supression, however,
it also might have the effect of reducing group delay distortion through
the SSB filter (a simple 4-pole job). In any event, the interesting thing
is that the audio processing is done with a single chip and just a few external
components.


 
 
 

Strange balanced modulator design question

Post by Richard Karlqui » Wed, 18 Sep 1996 04:00:00


>it also might have the effect of reducing group delay distortion through
>the SSB filter (a simple 4-pole job). In any event, the interesting thing
>is that the audio processing is done with a single chip and just a few external



4 pole filters have *less* delay distortion than higher order
ones (6 or 8 poles).

Rick Karlquist N6RK

 
 
 

Strange balanced modulator design question

Post by Marcus Ram » Wed, 18 Sep 1996 04:00:00


> I am looking at schematic of a Japanese SSB transceiver. It has a type of
> balanced modulator I've never seen before. The audio from the mic is
> amplified once by a conventional class A transistor amp, then fed into
> an IC. The IC has a 5 MHz crystal. Based on the block diagram, the
> purpose of the IC is to take 300-2700 Hz audio and shift it to 4400-2000
> Hz, before it is fed into a balanced modulator using four diodes.

        Well sir, I have an Elektrisk Bureau HF e***r (from Norway), dated 1973
that generates
AM-USB, USB and CAM using a similar aproach:

        1) The mic amplifier have a "several" dB per octave low-pass filter at
2.600Hz.
        2) This signal is applied to a balanced modulator which have a 20KHz
carrier.
        3) The "SSB" filter is a 19.7KHz low pass filter that just lets pass the
20KHz LSB signal.
        3) As professional xmtr like this, it has only USB/AM-USB at the output.
The four PLLs
           sum/differences places the audio signal on it's spectral USB position.

        To have LSB, what I did: another balanced modulator, with a 3.0KHz carrier
and a
        cascade of 2 op-amp filter, in a 7th order filter each, cut-toff abt
2.6KHz. So I got
        at this audio baseband the audio spectrum inverted - that's my LSB
solution. The only
        problem I have is that I need to set the e***r 3KHz lower in frequency
when I want to
        transmit in LSB. But the audio quality is impressive! Don't forget that my
original low-
        pass filter in the transmitter helps to remove the 3300...5900Hz
sideband...

Best Regards:
Marcus Ramos, PY3CRX